1. Field of the Invention
The present invention relates to a power unit having an internal combustion engine and a transmission and more particularly to an auxiliary machine driving transmission mechanism having an endless transmission belt wound around a drive sprocket and a driven sprocket to drive an auxiliary machine for the internal combustion engine.
2. Description of Background Art
As an example of an auxiliary machine driving transmission mechanism which drives an auxiliary machine for an internal combustion engine, one that has been disclosed in JP-A No. 2002-235548 (FIG. 6) has been known. The disclosed transmission mechanism includes a drive sprocket on the main shaft of a transmission and a chain wound around the drive sprocket in order to drive an oil pump as an auxiliary machine by rotation of the internal combustion engine. The crank shaft of the internal combustion engine, the main shaft as an input shaft for the transmission, and the counter shaft as an output shaft for the transmission are aligned along the vehicle's longitudinal direction and are parallel to each other on a separating plane which divides the crankcase into an upper case and a lower case, with the oil pump located below the separating plane.
In the above related art, since the main shaft on which the drive sprocket is placed lies on the crankcase separating plane, its distance from the oil pump located below the separating plane is short, eliminating the need for a guide for the chain. However, in the case that in order to reduce the longitudinal length of the power unit, composed of an internal combustion engine and a transmission, for the sake of compactness by compact longitudinal arrangement of the internal combustion engine and transmission, the center distance between the crank shaft and the counter shaft is shortened by placing the main shaft above and away from the separating plane, the distance between the drive sprocket on the main shaft and the oil pump located below the separating plane increases, which might cause vibration of the chain.